PROJECT SUMMARY Arsenic, a priority Superfund contaminant, neurotoxin, and carcinogen, is a ubiquitous metalloid contaminant polluting many urban freshwater ecosystems. However, the human health and ecological implications of this contamination are unclear due to an incomplete understanding of arsenic bioavailability in urban waters, which are typically affected by eutrophication. The objective of the proposed project is to quantify spatiotemporal patterns and primary drivers of arsenic mobility, bioavailability and ecological toxicity in urban lakes. The South-Central Puget Sound Lowland region offers an exceptional environment to study the human and environmental health impacts of urban metal(loid)-impacted freshwater ecosystems because it contains an array of densely populated lakes with arsenic-contaminated waters that display a wide range of redox behaviors: seasonally stratified and anoxic to well-mixed and oxic. Although elevated levels of arsenic usually occur in anoxic waters at the bottom of thermally-stratified lakes during the summer, at least one lake in the study region maintains elevated aqueous arsenic concentrations under oxic conditions. This situation raises questions about the physical and geochemical processes controlling arsenic chemistry in oxic waters of shallow, unstratified lakes, and also about the resulting bioavailability of arsenic to aquatic life, including fish. Due to limitations that anoxia poses for aquatic organisms, the typical coincidence of elevated arsenic and anoxic conditions may act to minimize biological exposure to arsenic, whereas shallow, unstratified lakes may enhance exposure to arsenic contamination. Within this context, the proposed project will pursue three specific aims: (1) determine the physical and biogeochemical conditions that promote arsenic mobilization from sediments and maintain elevated aqueous concentrations within shallow, unstratified oxic lakes, (2) identify the physical and chemical factors that control arsenic bioaccumulation through a lake food web in both stratified and unstratified lakes, and (3) assess ecological toxicity of arsenic within both stratified and unstratified lakes using established and novel molecular biomarkers (identified using RNA-Sequencing technology) indicating arsenic injury. We hypothesize that arsenic contamination in oxic waters of unstratified lakes is promoted and maintained by inputs of nutrients and organic matter, and that arsenic bioaccumulation and toxicity is enhanced in oxic unstratified lakes compared to seasonally stratified lakes. We will achieve our aims by linking measurements of physical, chemical and biological lake properties with investigations of arsenic bioaccumulation and ecological toxicity. Our proposed research is innovative because of its combined biogeochemical and eco-toxicological approach and its use of novel toxicogenomic methods to identify molecular biomarkers of arsenic injury. Our project is significant because it will provide information needed to establish effective water quality criteria, develop robust lake management strategies, and foster adoption of biomarker techniques as a way to monitor and assess ecological toxicity of arsenic in aquatic systems.